White spots on tomato plants are caused by psyllids, tiny insects about the size of aphids. The psyllids' young, which are even tinier than the adult psyllids, create the spots. The nymphs excrete tiny white grains that adhere to the leaves, usually on the undersides or in shaded areas.
Psyllid eggs and nymphs are difficult to spot. The young are a pale yellow when they hatch. As they eat, they turn green, thanks to the chlorophyll in the leaves. The nymphs typically stay in one spot as they feed and excrete the sticky white grains. During heavy infestations, the leaves may look as if they are dusted with sugar.
Psyllids usually start out on potato and pepper plants if available, since their leaves mature earlier than those of the tomato plant. Psyllids secrete a toxic saliva that can harm all three plant species. The saliva causes parts of the leaves to turn yellow and then curl. The top of the plant is most affected, giving it a ragged appearance.
Dusting the leaves with insecticides containing permethrin or sulfur can help control the problem. Psyllids survive year-round in the warmer parts of the United States, but die off in winter in snowier climates. The adults have wings, so they are usually blown into northern locales in the spring.